Generally, the term “hydrogel” refers generally to a polymeric material that is capable swelling in water. The swelling of a hydrogel in water results from diffusion of water through the glassy polymer causing disentanglement of polymer chains and subsequent swelling of the polymer network. Typically, hydrogels of the prior art have been prepared by the crosslinking of monomers and/or polymers by radiation, heat, reduction-oxidation, or nucleophilic attack. Examples of the crosslinking of ethylenically unsaturated monomers include the preparation of contact lenses from 2-hydroxyethyl methacrylate and the preparation of absorbent articles from acrylic acid. Examples of crosslinking of polymers include wound dressings by crosslinking aqueous solutions of hydrophilic polymers using ionizing radiation and surgical sealants by crosslinking aqueous solutions of hydrophilic polymers modified with ethylenically unsaturated moieties.
In or about 1968, Krauch and Sanner described a method of polymerizing monomers around a crystalline matrix and subsequently removing the crystaline matrix to produce an interconnected porous polymer network. Since that time, porous hydrogels have been prepared using salt, sucrose, and ice crystals as the porosigen. These porous hydrogels of the prior art have been used as membranes for affinity chromatography and as tissue engineering substrates wherein tissues are intended to ingrow into the porous hydrogel network. Examples of these porous hydrogels are found in U.S. Pat. No. 6,005,161 (Brekke, et al.) entitled Method And Device For Reconstruction of Articular Cartilage, U.S. Pat. No. 5,863,551 (Woerly) entitled Implantable Polymer Hydrogel For Therapeutic Uses and U.S. Pat. No. 5,750,585 (Park et al.) entitled Super Absorbant Hydrogel Foams.
The prior art has also included certain hydrogels that undergo a volume change in response to external stimuli such as changes in the solvent composition, pH, electric field, ionic strength, and temperature. The hydrogel's response to the various stimuli is due to the judicious selection of the monomer units. For example, if temperature sensitivity is desired, N-isopropyl acrylamide is frequently used. If pH sensitivity is desired, a monomer with an amine group or a carboxylic acid is frequently used. Stimuli responsive hydrogels have primarily been used as controlled drug delivery vehicles. Examples of these stimuli-responsive hydrogels are found in U.S. Pat. No. 6,103,865 (Bae, et al.) entitled pH-Sensitive Polymer Containing Sulfonamide And Its Synthesis Method, U.S. Pat. No. 5,226,902 (Bae et al.) entitled Pulsatile Drug Delivery Device Using Stimuli Sensitive Hydrogel and U.S. Pat. No. 5,415,864 (Kopeck, et al.) entitled Colonic-Targeted Oral Drug-Dosage Forms Based On Crosslinked Hydrogels Containing Azobonds And Exhibiting pH-Dependent Swelling.
Despite these advances in the capabilities of the hydrogel material, a hydrogel material that permits cellular ingrowth and has controlled rate of expansion optimized for delivery through a microcatheter or catheter without the need for a non-aqueous solvent or a coating has not been developed. Accordingly, there remains a need in the art for the development of such a hydrogel useable in various applications, including, but not limited to, medical implant applications wherein the hydrogel is used as or in conjunction with aneurysms, fistulae, arterio-venous malformations, and vessel occlusions.